Dr. Ion Alexandru Bobulescu is a physician-scientist recently appointed to a full-time faculty position at the instructor level at the University of Texas Southwestern Medical Center at Dallas (UT Southwestern). His short-term career goals (next 4 years) are to rigorously pursue the training and career development plan outlined in this K01 application, perform the proposed experiments using both established and highly innovative techniques to define the role of Protein Phosphatase 2A (PP2A) in the regulation of renal sodium/proton exchange, and then apply for funding under the R01 mechanism to explore one or more of the scientific opportunities derived from this work. Dr. Bobulescu's long-term career goal is to be a leader and innovator in the field of renal regulation of sodium and blood pressure homeostasis, using pioneering approaches and techniques to address fundamental unanswered questions from molecule to model organism to human disease. To achieve these goals, a thorough and detailed Career Development Plan is proposed to address the following training requirements: (1) COURSEWORK. Additional formal training in several areas is considered critical for the advancement of the candidate's research. (2) ADVANCED TECHNIQUES. Working with his mentor and expert collaborators on a hands-on basis to address the Aims of this proposal, Dr. Bobulescu will learn several novel, state-of-the-art methods, and will gain the ability to apply these techniques for his own research questions. (3) MENTORED CAREER DEVELOPMENT. Dr. Bobulescu's chances of achieving relevant results in the new and understudied field of signal transduction via protein phosphatases will greatly benefit from having an expert mentor who will directly guide, supervise and evaluate his research. The mentor for this K01, Dr. Marc Mumby, is a world-renowned expert in protein phosphatase research. In addition, Dr. Bobulescu is aware of the fact that a successful independent career requires much more than ideas, technical skills, and a good track record of publications. Running a research laboratory, managing and nurturing people, fostering collaborations, being an effective mentor and teacher, and being able to maintain a strong and productive research program resulting in successful funding beyond the first R01, are all formidable endeavors- but vital for true academic success. The candidate is certain that he has a lot to learn in these areas from Dr. Mumby, a leading scientist and ideal mentor for this K01. (4) DEPARTMENT. The disease-oriented perspective offered by Dr. Bobulescu's previous postdoctoral research training in a clinical department has been very important for his formation as a physician scientist. However, his training would greatly benefit from a period of immersive exposure to the vastly different environment of a leading basic science department. ENVIRONMENT: UT Southwestern is one of the world's foremost research institutions, with four Nobel prize winners, 36 members of the National Academy of Sciences and/or its Institute of Medicine, more than 3,500 research projects under way with more than $400 million in annual funding, as well as more than 582,000 sq ft of research space with more under construction. Dr. Bobulescu will train primarily in the laboratory of Dr. Marc Mumby in the Department of Pharmacology, including over 2,000 sq ft of total laboratory space, well equipped with all necessary items needed to carry out biochemical, molecular biological, and cell biological experiments. Dr. Bobulescu will also retain his 250 sq ft laboratory space, private 90 sq ft office and faculty position in the Department of Internal Medicine. The candidate will have access to UT Southwestern's state of the art animal research facility, as well as to the UT Southwestern George M. O'Brien Kidney Research Core Center. UT Southwestern has a tradition of retaining the best postdoctoral fellows and fostering their independent careers. This tradition includes the candidate's former mentor, Dr. Orson Moe, who was a postdoctoral fellow at UT Southwestern until 1990 and has remained there ever since, rising to full Professor and Center Director. RESEARCH: This proposal will study the role of PP2A in the regulation of the principal renal sodium transporter NHE3, with special emphasis on the regulation of NHE3 by intra-renal dopamine. If the proposed hypotheses are correct, PP2A will be established as a major regulator of intrarenal dopamine action and of NHE3. This will have a direct impact on our understanding of the physiology and pathophysiology of blood pressure regulation, since impaired dopamine-mediated natriuresis has been described in hypertensive humans, has been attributed to defective intra-renal dopamine signaling in two rodent models of hypertension, and has been linked to defective renal PP2A function. The proposal includes three independent aims that will be pursued simultaneously using a variety of techniques. Aim 1 will use cell culture and in vitro experiments to test whether the inhibition of NHE3 by dopamine requires the direct action of PP2A on NHE3. Aim 2 will use cell culture, biochemistry and advanced electrophysiological techniques to test whether dephosphorylation of NHE3 by PP2A acts as an intracellular switch determining the fate of NHE3. Aim 3 will use advanced in vivo imaging and immunohistochemistry to test whether the inhibition of NHE3 in vivo by physiologic stimuli involves NHE3 dephosphorylation by PP2A. By proposing to uncover the molecular details of a signaling pathway that, when impaired, has been implicated in the pathogenesis of hypertension, this project is clearly relevant to the mission of NIDDK to support research on the most serious diseases affecting public health.